Substrate bombardment by energetic ions is a central element of plasma etching used in fabrication of integrated circuits (IC), as well as plasma processes for thin film deposition and surface modification. A primary feature of plasma etching is etch directionality resulting from positive ions bombarding the substrate at normal incidence. For commercial IC fabrication, high etch rates, etch anisotropy, high etch selectivity between materials and low damage must be achieved simultaneously, and all are sensitive to ion bombardment energy, in addition to ion flux, gas phase chemical composition, substrate temperature, and other factors. Reduction of device dimensions and the use of new materials associated with continuing advancement in IC performance further constrains etch processes.
High etch selectivity, in which the etch rate of one material is much higher than that of others, is a necessary characteristic for many etch processes in integrated circuit (IC) manufacturing. Highly selective etching can prevent excessive erosion of photoresist and/or underlying films, thereby permitting overetching to compensate for across wafer etch non-uniformities. For example, as part of the “damascene” process, plasma etching is used to etch trenches and vias in inter layer dielectric (ILD) layers, which are then filled with metal to form the interconnects between transistors and other IC components. In order to precisely control the depth of these trenches, thin etch stop layers (ESLS) of a different dielectric material may be introduced at the desired depth. The success of this approach depends on a plasma etch process with sufficiently high etch selectivity over the ESL material.
In addition to selectivity, substrate damage induced by bombardment by high energy ions is another consideration in etching processes. As IC feature sizes continue to shrink, the thickness of the gate oxide is expected to decrease to only a few tens of angstroms in the near future. As a result, the gate oxide becomes more vulnerable to high energy ion bombardment and cannot tolerate any thinning during overetch. Further, when the line width of photoresist becomes thinner, it becomes less resistant to erosion by sputtering. Therefore, more precise control of ion bombardment energy at the substrate may play a key role in future IC fabrication, especially for processes with poor selectivity.
A third area sensitive to ion bombardment energy is the control of the profile of high aspect ratio features etched into the substrate using a plasma process. Local charging of feature sidewalls by electrons during the etching process may affect the shape of the feature as it develops by causing deflection of the trajectories of bombarding ions. In the etching of high aspect ratio trenches, for example, a combination of both low and high energy ions may be desirable, as the low energy ions entering the trenches may be easily deflected to neutralize negative charge on the trench sidewalls due to incident plasma electrons, minimizing deflection of higher energy ions which can then reach the trench bottom to enhance etching there.
In general, there are two possible components to any approach to increasing etch selectivity, chemical and physical. The chemical component involves making use of the influence of chemically different ion and neutral species in the plasma, by changing the gas mixture or operating conditions to improve selectivity. Etching of silicon and silicon dioxide in high density plasma process tools, for example, has shown higher SiO2/Si selectivity for feed gases with high carbon to fluorine atom ratios, such as C2F6 and C4F8 or by the addition of H2. Alternatively, radical densities in plasmas may be indirectly controlled by adjusting the chamber wall temperature or injecting radical species. In all of these cases, the selectivity of SiO2/Si is based on the selective deposition of fluorocarbon films on silicon.
The physical component involves controlling the bombardment of the substrate by energetic ions. If most ions bombarding the substrate strike with energy above the etch threshold for SiO2, or any other material of interest, and below those of the other materials, the etch selectivity will be infinite. In typical plasma processes used to manufacture ICs, the ion energy is coarsely controlled by varying the amplitude of a radio frequency (RF) sinusoidal bias voltage that is applied to the substrate electrode. However, the resulting ion energy distribution function (IEDF) is generally broad, which limits the ability of the plasma process to further improve etch selectivity. By replacing the standard sinusoidal waveform with a waveform that produces an appropriate energy distribution of bombarding ions, greater etch selectivity can be achieved.
Thus, accurate and reproducible control of ion bombardment energy is desirable to control process outcomes. IED control can be achieved by tailoring the waveform shape of the RF bias voltage applied to the substrate during processing using a programmable waveform generator in combination with a power amplifier. Initial studies of bombarding ion energy control involved a manual trial and error method of setting the voltage waveform on the substrate by adjusting the voltage waveform output of the signal generator. One cycle of the generator output was programmed point by point (about 85 points per cycle) and modified until the desired substrate waveform was achieved. This approach limits control of electrode waveform shape as well as the ability to create more complex shapes, because the process is unpredictable and therefore lengthy and tedious. Additionally, due to the frequency dependence of the amplifier gain and the impedance of the plasma in contact with the substrate as well as other system nonlinearities, it is not practical to try to predict the shape of the waveform needed at the waveform generator output to produce a target waveform voltage at the substrate. Thus, a method and a system for automatically producing a target waveform at the substrate is needed.